Speakers

Biomimetic Intermediates in Dioxygen Activation

The dioxygen activation at the catalytic sites of the enzymes occurs through several steps including the binding of O2 at a reduced metal center, the generation of metal-superoxo and -peroxo species, and the O-O bond cleavage of metal-hydroperoxo complexes to form high-valent metal-oxo oxidants. Because these mononuclear metal-dioxygen (M-O2) adducts are implicated as key intermediates in dioxygen activation reactions catalyzed by metalloenzymes, studies of the structural and spectroscopic properties and reactivities of synthetic biomimetic analogues of these species have aided our understanding of their biological chemistry. In this presentation, we report the synthesis, structural and spectroscopic characterization, and reactivity studies of M-O2(H) complexes bearing tetraazamacrocyclic pyridinophane ligands. Different steric properties of the supporting ligands control the reactivity of the intermediates. In addition, we investigate the reactivity comparison of M-O2 and -O2H species toward organic substrates.

A mononuclear side-on peroxocobalt(III) complex with a tetradentate macrocyclic ligand, [CoIII(TBDAP)(O2)]+ (1), also shows a novel and facile mode of dioxygenase-like reactivity with nitriles (R-CºN; R = Me, Et, and Ph) to produce the corresponding mononuclear hydroximatocobalt(III) complexes, [CoIII(TBDAP)(R-C(=NO)O)]+, in which the nitrile moiety is oxidized by two oxygen atoms of the peroxo group. The overall reaction proceeds in one-pot under ambient conditions (ca. 1 hour, 40 °C). 18O-labeling experiments confirm that both oxygen atoms are derived from the peroxo ligand. The structures of all products, hydroximatocobalt(III) complexes, were confirmed by X-ray crystallography and various spectroscopic techniques. Kinetic studies including the Hammett analysis and isotope labeling experiments suggest that the mechanistic mode of 1 for activation of nitriles occurs via a concerted mechanism. This novel reaction would be significantly valuable for expanding the chemistry for nitrile activation and utilization. Since the spin crossing between S = 0 and S = 1 for 1 was observed, another interesting point is that the spin state of 1 may play an important role in the nitrile group activation.